1. Field of the Invention
The present invention relates to a boundary acoustic wave device using a Stoneley wave, and more particularly, to a boundary acoustic wave device using a Stoneley wave and including electrodes that are disposed at a boundary between a piezoelectric substance and a dielectric substance.
2. Description of the Related Art
Heretofore, various surface acoustic wave devices have been used for RF and IF filters in mobile phones, resonators in VCOs, VIF filters in televisions, and other devices. Surface acoustic wave devices use a surface acoustic wave, such as a Rayleigh wave or a first leaky wave, which propagates along a surface of a medium.
Since the acoustic wave propagating along a surface of a medium, a surface acoustic wave is sensitive to changes in the surface condition of the medium. Accordingly, in order to protect a surface of a medium along which a surface acoustic wave propagates, a surface acoustic wave element is often hermetically sealed in a package in which a cavity portion is provided so as to face the wave-propagating surface. Since the package having a cavity portion as described above is used, the cost of the surface acoustic wave device is increased. In addition, since the size of the package is much larger than that of the surface acoustic wave element, the size of the surface acoustic wave device is increased.
On the other hand, among acoustic waves, in addition to the above-described surface acoustic waves, a boundary acoustic wave propagates along a boundary between solid substances.
For example, in “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrasonics, VOL. SU-25, No. 6, 1978 IEEE (non-patent document 1), a boundary acoustic wave device is disclosed in which an IDT is disposed on a 126° rotated Y plate X-propagation LiTaO3 substrate and a SiO2 film having a predetermined thickness is disposed on the LiTaO3 substrate and the IDT. In non-patent document 1, an SV+P type boundary acoustic wave, which is a so-called Stoneley wave, is propagated. In non-patent document 1, when the thickness of the above SiO2 film is set to 1.0λ (λ indicates the wavelength of a boundary acoustic wave), an electromechanical coefficient of 2% is obtained.
The boundary acoustic wave propagates in the state in which energy is concentrated on a boundary portion between solid substrates. Hence, since energy is not substantially present on the bottom surface of the above LiTaO3 substrate and the surface of the SiO2 film, the properties are not changed by changes in surface conditions of the substrate and the thin film.
Accordingly, a package having a cavity portion is not required, and hence the size of the boundary acoustic wave device is reduced.
In addition, in “Piezoelectric Boundary Wave in a Substrate with a Layered Structure” authored by Chujo, Yamanouchi, and Shibayama, Research Institute of Electrical Communication, US80-4, 1980 (non-patent document 2), a boundary acoustic wave called a Stoneley wave is disclosed which propagates in the structure in which a SiO2 film is disposed on a 128° rotated Y plate X-propagation LiNbO3 substrate. According to the analysis of the non-patent document 2, it is shown that when the SiO2 is in its natural state, since the displacement is not concentrated on the boundary between the SiO2 layer and the LiNbO3 substrate, a boundary acoustic wave is not generated, and that when the Lame constant indicating the elasticity of SiO2 is changed from an inherent value of 0.3119×1011 N/m2 to 0.4679×1011 N/m2, the displacement is concentrated on the boundary, such that a boundary acoustic wave is generated. In addition, according to the experimental result of the non-patent document 2, it has also been disclosed that even when conditions for forming the SiO2 layer are changed, a SiO2 film cannot be formed in which a boundary acoustic wave propagates.
In a boundary acoustic wave device, a large electromechanical coefficient, a small propagation loss, a small power flow angle, and a small temperature coefficient of frequency have been required. The loss caused by the propagation of a boundary acoustic wave, that is, the propagation loss, may degrade the insertion loss of a boundary acoustic wave filter or may also degrade the resonant resistance or the impedance ratio of a boundary acoustic wave resonator, the impedance ratio being a ratio between the impedance at a resonant frequency and that at an antiresonant frequency. Hence, the propagation loss is preferably decreased to as small as possible.
The power flow angle is an angle indicating the difference between the direction of the phase velocity of a boundary acoustic wave and the direction of the group velocity of energy thereof. When the power flow angle is large, it is necessary to obliquely dispose an IDT in conformity with the power flow angle. Hence, the design of the electrodes is complicated. In addition, a loss caused by a deviation in the angle is likely to be generated.
Furthermore, when an operating frequency of a boundary acoustic wave device is changed by the temperature, practical pass band and stop band are decreased in a boundary acoustic wave filter. With a resonator, when an oscillation circuit is formed, the above-described change in operating frequency caused by the temperature results in abnormal oscillation. Hence, the change in frequency per degree centigrade, which is TCF, is preferably decreased to as small as possible.
For example, when reflectors are disposed along a propagation direction and outside a region in which a transmitting IDT and a receiving IDT are provided, which transmits and receives a boundary acoustic wave, respectively, a low-loss resonator type filter can be formed. The band width of this resonator type filter depends on the electromechanical coefficient of a boundary acoustic wave. When the electromechanical coefficient k2 is large, a broadband filter is obtained, and when the electromechanical coefficient k2 is small, a narrowband filter is obtained. Thus, the electromechanical coefficient k2 of a boundary acoustic wave used for a boundary acoustic wave device must be appropriately determined in accordance with its application. When an RF filter for mobile phones is formed, the electromechanical coefficient k2 is required to be at least 5%.
However, in the boundary acoustic wave device using a Stoneley wave, which is disclosed in non-patent document 1, the electromechanical coefficient k2 is small, such as 2%.
In addition, in the SiO2/LiNbO3 structure disclosed in the non-patent document 2, a LiNbO3 substrate having large piezoelectric properties is used. Hence, compared to the boundary acoustic wave device described in the non-patent document 1, a larger electromechanical coefficient k2 can be obtained. However, it is difficult to form a SiO2 film such that a boundary acoustic wave propagates, and the non-patent document 2 discloses no measurement results of a Stoneley wave after actually forming the SiO2 film.